1. Technical Field
The present invention relates generally to combustion gas turbine engines, and more particularly, to an active control system for controlling the blade tip clearance of a combustion gas turbine engine.
2. Description of the Related Art
The efficiency of a combustion gas turbine engine is dependent upon many factors, one of which is the radial clearance between adjacent rotating and non-rotating or stationary components such as between the blade tips and the ring segments that are circumferentially mounted on a blade ring and are disposed adjacent the blade tips. If the clearance is too great, an unacceptable degree of gas leakage will occur with a resultant loss in efficiency. If the clearance is too little, a risk exists that under certain conditions undesirable physical contact will occur between the rotating and stationary components.
Prior to operation of the engine, an initial clearance exists between the rotating and stationary components of the engine. When the engine is initially started, the clearance decreases due to centrifugal forces and thermal growth of the rotating components. In this regard, it is understood that the rotating components initially tend to heat up and thus thermally grow at a faster rate than the stationary components. Nevertheless, inasmuch as the stationary components are circumferentially large, the thermal growth that is eventually experienced by the stationary components is substantially greater than that experienced by the rotating components. As such, during engine startup the blade tip clearance initially decreases until the stationary components heat up and begin to experience their own thermal growth, which has a tendency to increase the blade tip clearance.
It can be seen , therefore, that during engine startup the blade tip clearance decreases from the initial clearance to a minimum clearance, and thereafter increases until the engine reaches steady state operation, after which the engine operates at a constant running clearance. The minimum clearance point is known as the xe2x80x9cpinch pointxe2x80x9d of the engine, meaning that the rotating components are at their closest proximity with the stationary components. Inasmuch as it is desired to avoid physical contact between the rotating and stationary components, engines must be designed around the pinch point to ensure that no such contact occurs during operation of the engine.
While different types of control systems have been proposed in an attempt to alleviate the running clearance that occurs during steady state operation of the engine, a need nevertheless exists for an active control system that avoids the pinch point of the engine to thereby improve performance. Additionally, known control systems typically employ adjustable flow impediments which adjust the rate at which bleed air is delivered to certain components of the engine. Such variability in the rates of bleed air flow has a detrimental effect on engine efficiency. A need thus exists for an active control system that controls the temperatures of engine components without adjusting the flow rates at which bleed air is delivered to the components. Additionally, no such control system has employed a sensor that continuously monitors the blade tip clearance and allows for corrective signals to maintain the engine at a desired tip clearance and efficiency. A need thus exists for an active control system that performs such continuous monitoring and allows for such continuous correction.
A method of actively controlling the clearance between the rotating components and the stationary components of a combustion gas turbine engine includes employing a control system that controls the temperature of bleed air that is delivered to the stationary and rotating components to control the thermal growth thereof and to avoid a pinch point. The control system includes one or more sensors that are circumferentially distributed about the engine and measure the blade tip clearance. The clearance measurements are directed to a controller that generates a correction signal corresponding with a desired clearance setting. The correction signal controls the operation of heat sources interposed within the air passages that deliver bleed air to the stationary and rotating components. The heat sources supply heat to the bleed air at specified rates responsive to the correction signal to control the thermal growth of the stationary and rotating components and to control the blade tip clearance. The use of multiple sensors permits the system of the present invention to alleviate the negative effects of xe2x80x9covalizationxe2x80x9d of the stationary components that often occurs with use of the engine due to plastic deformation and that results in the stationary components creeping from a circular configuration to a non-circular or oval-shaped configuration.